There was no shortage of instrumental guitar records in the late 50s and 60s. In the UK there were the Shadows, while in the US groups such as the Ventures had a string of hits, with the surfing craze spurring myriad garage bands to have a stab at a chink of the limelight.

But Sleep Walk isn't like the others. Its funereal pace, that unexpected minor-fourth chord and the otherworldly steel guitar still melts the heart more than 50 years since its release in 1959. A US No 1, it propelled its young makers Santo and Johnny into the top flight of the rock'n'roll era.

Santo Farina retired from music in the early 70s. But his brother Johnny Farina, who accompanied him on standard electric guitar, continues to perform, and spoke to me by phone from his home in Long Island, about 50 miles from where the pair grew up in Brooklyn. He says they owed their career to their father, who had discovered the steel guitar during the war, listening to country and western music while he was stationed in Texas and Oklahoma: "Our father really had no musical talent except to listen to the radio. He fell in love with the steel guitar. He told my mom to have the boys learn how to play this beautiful instrument."

But the slide guitar was something of an exotic anomaly in the late-50s Italian-American neighbourhood. "We were exposed to mandolins, acoustic guitars and accordians. So for us to take lessons on a steel guitar … No one really wanted to hear it. Everyone was doing doo-wop. They associated the steel guitar sound with country and Hawaiian music. Although really, the way we play it is not Hawaiian at all."

Santo and Johnny's father also encouraged the brothers to write their own music. "We would play cover tunes and our original songs. We got a really big following in Brooklyn. We would be playing at church dances and clubs. One night after we finished playing – we were so excited from playing that we couldn't sleep – we started to write Sleep Walk. My father had bought us a Webcor tape recorder, and we recorded everything we wrote. We just created songs, and didn't think much of it."

Farina immediately recognised the song's potential – but had a job convincing others of its worth. "When we did Sleep Walk I was 16. My brother, who's four years older than me, had no desire to proceed any further than playing clubs at the weekend. I kept telling him we have a special sound, and people will love this. He didn't want to pursue it. But I went around for a year and a half until I finally got somebody interested in listening to the song."

Sleep Walk took off when it was played by Alan Freed, the DJ who claimed to have coined the term rock'n'roll. "He said: 'This is going to be a monster hit! This thing will live forever,'" Farina says. "And he was right."

Even after all this time, Farina finds it hard to explain Sleep Walk's special appeal: "When people hear it, you can hear a pin drop. That melodic sound … it's just a magical bunch of notes put together with certain feeling. We had no idea we were creating such a Mona Lisa."

Like the enigmatic smile on the Mona Lisa, many myths have grown around the origins of Sleep Walk. What about the third Farina who is credited with writing the song, Santo and Johnny's sister Ann? "We have no sister Ann! Actually, Ann was Santo's wife. Her name was put on there by mistake. I keep getting the same question: 'So, did your mom write the record? Did your sister write the record?' We have no sister. My mom's a good cook – that's about it, you know?"

And is there anything in the claim that Sleep Walk is derived from a 1928 song called Softly As in a Morning Sunrise? "I heard about that song after Sleep Walk came out, some years later. It's not even close to it, really, if you listen to the two. But it's become part of the mystique of the song."

Santo and Johnny's distinctive sound influenced a generation of musicians celebrated for pioneering new music. "Sleep Walk's in everybody's DNA," says Farina, who's keen for his place in rock history to be more widely recognised. "John Lennon said he was inspired by Sleep Walk, and that's why he wrote Free as a Bird. George Harrison released a song called Marwa Blues inspired by Santo and Johnny. Also he loved All Night Diner, the flip side of Sleep Walk. When I heard George Harrison was so inspired – these people in the UK, like Jimmy Page and George Harrison – I didn't know we touched so many people."

When I mention Santo's decision to quit the music business, Farina bristles. "I never knew why Santo wanted to retire. I told him, it's crazy to retire. But he just didn't want to do it. I kept going. I think when people hear Sleep Walk, Santo and Johnny – they must think we're like, 200 years old, those guys are not around. I would like people to know – I'm around! I'm still doing shows! It looks like I'll be in the UK in November. I'll be in France in April."